Vapor storage canisters, such as carbon canisters, are used in vehicles to reduce vapor emissions caused by temperature and/or pressures changes in the fuel tank. For instance, temperature shifts in the fuel tank which may be caused by diurnal cycles, heat rejection from underbody components such as an exhaust pipe, and/or hot return fuel from the engine can generate fuel vapors in the fuel delivery system. Fuel vapor may also be generated during refueling because of air entrainment with liquid fuel, turbulence, and temperature differences between tank fuel and fresh fuel. Furthermore for hybrid vehicles, the fuel tank is sealed at high pressure. This pressure is released rapidly during refueling. This pressure change can also cause vapor generation. The fuel vapors may leak or permeate from the fuel tank if not properly sequestered. Therefore, in some vehicles fuel vapors are routed to carbon canisters for temporary storage to reduce emissions. The fuel vapors may be subsequently purged during certain operating conditions to prevent overfilling of the vapor storage canister. During purging operation, fresh air may be introduced into the canister causing desorption of the fuel vapors from the carbon in the canister. Then, the mixture of air and fuel vapor is routed into engine via an intake system where they are combusted.
U.S. Pat. No. 8,246,729 discloses a fuel vapor storing device having a tubular diffuser with plurality of openings providing air into the device during purging. However, the fuel vapor storing device disclosed in U.S. Pat. No. 8,246,729 does not provide a desired amount of flow distribution in the device during purging. Specifically, the tubular diffuser may not generate flow patterns which evenly distribute the airflow through the device when purged. The tubular/annular diffuser described in aforementioned patent also increases pressure drop across canister because of narrow flow passages and flow turning. As a result, the desorption rate of fuel vapor into the intake air may be decreased during periods of high inlet airflow. Consequently, there may be trade-offs between purging efficiency (e.g., the amount of fuel vapor purged from the canister per volumetric airflow) and the flow-rate of air during purging. As a result, a desired amount of fuel vapor may not be purged from the device in a desired period of time, preventing the device from being completely purged. Consequently, the device may reach maximum vapor storage, thereby increasing fuel vapor emission from the vehicle. This may be particularly problematic in plug-in electric hybrid vehicles (PHEV) where high purge rates are desired due to the limited window of engine combustion operation in the vehicle.
The inventors herein have recognized the above issues and developed systems and method for addressing the issues. In particular, a mixing valve is disclosed which may be positioned upstream of a fuel vapor canister for improving purging efficiency of the canister. In one example, a system for an engine may comprise a fuel vapor canister, a mixing valve positioned in a fresh air line upstream of the vapor canister, and an actuator physically coupled to the mixing valve for adjusting a position of the mixing valve to increase turbulence in air entering the vapor canister. In some examples, the mixing valve may be adjustable between a closed first position where air does not flow past the mixing valve, and an open second position where air does flow past the mixing valve, where an amount of turbulence in air entering the vapor canister may increase with increasing deflection of the mixing valve towards the closed first position and away from the open second position.
The position of the mixing valve may be adjusted based on an amount of fuel vapor desorption from the fuel vapor canister, where the amount of vapor desorption may be determined based on outputs from an oxygen sensor positioned downstream of the canister between the canister and an intake manifold of the engine. Specifically, the position of the mixing valve may be adjusted to increase turbulence in air entering the vapor canister in response to decreases in the amount of vapor desorption. In other examples, the position of the mixing valve may be additionally or alternatively be adjusted to increase turbulence in air entering the canister in response to one or more of decreases in an intake manifold vacuum, opening of a throttle, and decreases in an airflow rate through the canister.
In another representation, an engine system may comprise an engine including an intake manifold, a fuel vapor canister fluidically coupled to the intake manifold via a purge line for purging fuel vapors thereto, a fresh air line fluidly coupled to the canister and open to ambient air for drawing said ambient air into the canister during purging of the canister, the fresh air line comprising two parallel conduits fluidically separated by a wall, a first mixing valve positioned in one of the conduits of the fresh air line, and a controller with computer readable instructions for adjusting a position of the mixing valve during purging of the canister to increase flow uniformity in the canister in response to outputs received from an oxygen sensor positioned in the purge line. In a first example of the engine system, the engine system may further comprise an actuator which may be in electrical communication with the controller and may be physically coupled to the mixing valve for adjusting the position of the mixing valve in response to signals received from the controller. In a second example of the engine system, the engine system may include one or more or each of a second mixing valve positioned in the purge line downstream of the canister, for increasing an amount of turbulence in air entering the intake manifold from the purge line.
In this way, an amount of fuel vapor desorption and therefore canister purging efficiency may be increased by adjusting a position of a mixing valve coupled in a fresh air line upstream of a fuel vapor canister. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the above summary is provided to introduce a selection of concepts in simplified form. These concepts are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Additionally, the above issues have been recognized by the inventors herein, and are not admitted to be known.